1. Field of the Invention
The present invention relates to a color difference line sequential circuit used in a magnetic recording system and, in particular, to a color difference line sequential ciruit which eliminates the effects of a clamping operation peformed when a color difference signal is line sequentialized in a recording system.
2. Description of the Prior Art
In a conventional color difference line sequential circuit employed in a recording system of a magnetic record/reproduction system in an electronic still camera or the like in which an image signal is frequency modulated and recorded in a magnetic recording medium and the recorded signal is then reproduced and displayed on the screen of a display device, there is a problem that effects resulting from a clamp operation performed in line sequentializing a color difference signal destroy a color balance in a reproduction system to deteriorate the quality of the image displayed on the screen. This problem will discussed below in connection with FIGS. 3 through 6. In FIG. 3, there is shown a general construction of a color difference line sequential circuit in a recording system of a conventional magnetic recording/reproducing system, and, in FIG. 4, there is shown a general construction of a color difference line sequential synchronization circuit in a reproducing system thereof. In FIG. 3, there are provided clamp circuits 10, 20 which are adapted to clamp the direct current levels during the blanking periods of color difference signals R-Y, B-Y which are input from terminals 100, 102, respectively. Clamp Circuit 10 comprises a capacitor C1, a switch 12, and a d.c. supply 14,while Clamp Circuit 20 comprises a capacitor C2, a switch 22, and a d.c. supply 24. These switches 12, 22 are analog switches, each of which is operated by a clamp pulse CP output for each horizontal scan period (1 H) from a pulse generation circuit 52. Also, there is provided a synchronization switch 30 is an analog switch which is adapted to take in and synchronize alternately and selectively for 1 H the color difference signals R-Y, B-Y respectively output from Clamp Circuits 10, 20, and this switch 30 is operable by a line pulse LP output from Pulse Generation Circuit 52. Referring further to FIG. 3, numeral 32 designates a pre-emphasis circuit, and 34 represents a frequency modulator.
In the above-mentioned color difference line sequential synchronization circuit in the reproduction system, as shown in FIG. 4, a line sequential color difference signal C input from a terminal 106 is divided into two signals: that is, an undelayed signal which is passed as it is, or, undelayed through an amplifier 60 and a clamp circuit 110 and is output to a synchronization switch 74; and, a delayed signal which is delayed by 1 H through 1 H delay line 62, is then passed through an amplifier 64 and a clamp circuit 120, and is output to to a synchronization switch 76. Clamp Circuit 110 comprises a capacitor C3, an analog switch 66 operable by Clamp Pulse CP, and a d.c. supply 70, while Clamp Circuit 120 comprises a capacitor C4, an analog switch 68 operable by Clamp Pulse CP, and a d.c. supply 72. In FIG. 4, reference numeral 78 denotes a color encoder which takes in the color difference signals R-Y, B-Y divided by Synchronization Switches 74, 76 operable by Line Pulse LP and converts the color difference signals into an NTSC signal 80.
In the above-mentioned construction, the color difference signals R-Y, B-Y are input at timings shown by (a), (b) in FIG. 5 to the terminals 100, 102 of the color difference line sequential circuit in the recording system, respectively, and the levels of direct current during the blanking periods of these color difference signals are clamped by Analog Switches 12, 22 operable by Clamp Pulse CP (FIG. 5(c)) which is output every 1 H, (FIG. 5(d)). During this clamping operation, since only a portion of the direct current levels in the blanking periods of the color difference signals is clamped to a given potential which is determined by D.C. Supplies 14, 24, there is produced a phenomenon (in this embodiment, for convenience' sake, this is referred to as a clamp disfigurement.) that a portion of Clamp Pulse CP is superposed on the d.c. level in the blanking period of the color difference signal (R-Y or B-Y) to be clamped, (see portions n1.about.n4 shown by oblique lines in FIG. 5(d)). Then, the color difference signals R-Y, B-Y affected by such clamp pulse CP are line sequentialized at timings of the line pulse (FIG. 5(f)), (see FIG. 5(e)).
On the other hand, in the reproducing system, the undelayed signal (FIG. 6(a)) of the line sequential color difference signal C input from the terminal 106 is input to Clamp Circuit 110 via Amplifier 60, while the 1 H delayed signal (FIG. 6(b)) obtained via 1 H Delay Line 62 is input to Clamp Circuit 120 via Amplifier 64. Here, after the line sequential color difference signal is synchronized, in order that the offset amounts of the d.c. levels of the line sequential color difference signal corresponding to the blanking periods of the undelayed and 1 H delayed signals before the line sequential color difference signal is synchronized are made to be equal to each other, the supply voltage of the d. c. supply 72 in Clamp Circuit 120 is regulated to thereby make the clamp levels in the blanking periods of the color difference signal be equal to each other in the undelayed signal and the 1 H delayed signal thereof.
However, in the before-described conventional system, there is left the clamp disfigurement in recording (FIG. 6(a), (b)). As a result of this, if such line sequential color difference signal is clamped by Analog Switch 66, 68 at the timing of the clamp pulse CP (FIG. 6(c)), then the clamp disfigurement portion in the blanking period of the line sequential color difference signal is clamped. In other words, if a voltage applied to the base line of the clamp disfigurement portion is expressed by .DELTA.V, an offset error of the direct current level corresponding to .DELTA.V appears on the color difference signal synchronized by synchronization switches 74, 76 operable by the line pulse (FIG. 6(d)), (see FIG. 6(e), (f)). Thus, the color difference signal having such offset error is, as it is, modulated by a color encoder 78, (FIG. 6(g)), resulting in the color blurs in the color signal.
As mentioned above, in the recording system in the conventional magnetic recording/reproducing system, there is a problem that the color blurrs are produced due to the clamp operation which is performed when the color difference signal is line sequentialized, resulting in the deteriorated quality of the picture.